According to the Czochralski (CZ) method, square crystals are pulled out of a crucible in the <100> direction if the radial temperature gradient in the melt was sufficiently small. Such a procedure, however, is unstable, that is to say, the cross section of the crystal changes very easily in response to small variations in the temperature. Moreover, due to the small temperature gradients, only low rates of pulling are possible. Additionally, the levels of impurities of CZ Si are higher than those of float-zone (FZ) silicon.
Square or virtually square single-crystalline wafers are manufactured, for example, for solar cells, in that segments having a round crystal cross section are cut off in a process that entails material losses of up to 36%.
The axis of rotation of a growing FZ crystal which, for process-related reasons, stands on its “bottleneck”, is not the main axis of inertia if its length exceeds the diameter. Consequently, its rotational movement is unstable for physical reasons. Particularly in the case of the large crystals (e.g. diameter of 150 mm, length of 1.5 m) commonly found in actual practice, deflections of the crystal and of the melt—triggered by small disturbances that are always present (vibrations, unbalances)—increase as the rate of rotation rises, contributing to the occurrence of dislocations or limiting a further increase in the diameter or pulling length of the crystal.
There are numerous solutions for the production of zone-pulled single-crystalline ingots having a round cross section.
For instance, East German patent application DD 263 310 A1 describes a method in which influences stemming from the forced convection are eliminated in that the crystal rotation and the rotating magnetic field have the same direction of rotation.
A method in which the rotational directions of the single crystal and of the magnetic field are opposite to each other is described in German patent application DE 100 51 885 A1. Here, a volume force is exerted in the azimuth direction in the melt, as a result of which a special flow is formed in the melt that brings about a longer residence time of the particles in the melt and thus a thorough mixing and complete melting. This process can be improved by using a second magnetic field with a different frequency and with an amplitude that changes over time.
In the method that is described in German patent application DE 36 16 949 A1 for the production of round single-crystalline semiconductor ingots, in addition to the high-frequency alternating field generated by the Hf heating coil, a magnetic direct-current field running parallel to the pulling axis is applied using a cylindrical coil that surrounds the melt zone and the Hf heating coil, as a result of which an eddy current damping of the convection of the melt is supposed to be achieved.
Also in the case of the solution described in U.S. Pat. No. 5,556,641 for the production of round single-crystalline Si ingots employing the FZ method, in which the polycrystalline Si ingots have an average particle size ranging from 10 μm to 1000 μm, additional means that generate a static magnetic field are employed, as a result of which eddies in the melt are supposed to be suppressed. For the growth procedure, the initial crystal ingot and the growing single crystal ingot are rotated.
For purposes of doping zone-pulled semiconductor material having a round cross section, a doping agent is added to the melt, which is described, for example, in German patent application DE 102 16 609 A1. The melt containing the doping agent is exposed to at least one rotating magnetic field. When the melt solidifies, the single crystal that is formed is rotated at a speed of at least 1 rpm and the magnetic field is rotated in the opposite direction.
The industry has an interest in producing Si wafers in a manner that saves material. The material loss encountered when round Si disks are cut into wafers having a certain cross section is no longer acceptable.
German patent application DE 36 08 889 A1 describes a method for the production of single-crystalline semiconductor ingots having a polygonal cross section employing the Czochralski crystal-growing method in which a defined temperature field that corresponds to the symmetry of the growing crystal is applied to the surface of the melt. To this end, a cooling system is provided that rotates in the same direction and at the same speed as the seed crystal. As a result of the controlled temperature distribution, crystal growth that differs from the usual cylindrical shape is attained.
German patent application DE 102 20 964 A1 describes a solution for the production of polycrystalline crystal ingots having a defined cross section through continuous floating-zone crystallization, whereby, by means of the adjustable distances between the induction coil and the crucible as well as between the induction coil and the crystallization front, a single shared heating means, namely, the induction coil, is used to melt the crystal material in the crucible and to discontinue the crystallization front on the growing polycrystalline crystal ingot. In this manner, Si particles that have not melted are not supposed to reach the phase boundary either. A frame that touches the melt and that is arranged right above the growing crystal ingot defines the shape, namely, the cross section, of the growing crystal ingot.